(1) Field of the Invention
The invention relates to ceilings capable of surviving seismic disturbances. The ceilings are of the suspended type that have a grid of interconnected metal beams hung from a structural ceiling. Panels are supported in the grid openings.
(2) Description of the Related Art
Suspended ceilings have a grid of beams hung from a structural ceiling by wires. Panels are supported in the grid openings. An angle wall molding runs around the perimeter of the ceiling and supports the ends of the grid beams. Such a ceiling is shown in U.S. Pat. No. 4,827,681, incorporated herein by reference.
The beams are roll formed of strip steel bent, in cross section, into a vertically extending web, horizontally extending flanges at the bottom of the web, and a bulb at the top of the web. The beams are interconnected into a grid. The grid is hung with wires from the structural ceiling, within the vertical walls of the individual rooms of a building. Such walls include structural walls as well as interior partition walls. A wall molding having an angle cross section supports the ends of the beams of the grid. An upper extending leg of the angle forms a face that is fastened against the wall, and the horizontal extending leg of the angle forms a ledge which receives and supports the beams. The end of the beam may simply rest on the ledge, or the beam flange may be secured to the ledge as by a self-tapping screw. The width of the ledge is usually about equal to the width of the beam flange, so, when viewed from below, the visual effect is a pleasing one.
In some instances, a perimeter clip is used to secure the ends of the beams to the wall molding. Such a clip is shown in U.S. Pat. No. 5,046,294, incorporated herein by reference. Such a perimeter clip is fixed on the wall molding at each end of a line of beams, and an end of a beam at the end of a line is fixed in a clip.
In a ceiling approved for an installation where a seismic event may occur, it is required that the beams in a grid be allowed to move relative to the side walls. Wires and the angle wall molding were relied on to keep the grid of beams supported in the ceiling, while allowing the beams to slide on the wall molding. To permit a beam to shake while supported on a ledge of a wall molding, seismic building codes often require a minimum two inch ledge on the wall molding to permit the beam end to slide on the ledge, without sliding off the ledge, during a quake. In such seismic codes, one end of a line of connected beams in the grid is required to be fixed to the wall molding; the other end must be free to slide back and forth longitudinally of the line of beams on the two inch ledge. Such a two inch wide ledge, however, is visually incompatible with the rest of the grid ceiling, when viewed from below, since the flanges on the beam are generally less than 1 inch wide.